The X-ray CT scanner is an apparatus provided with a pair of an X-ray tube and an X-ray detector arranged in a manner being opposed to each other (hereinafter, referred to as “imaging system”), placing a test subject therebetween, taking an image of transparent X-ray data of the test subject while turning the imaging system, and reconstructing a cross-sectional image (hereinafter, referred to as a “CT image”) of the test subject, and this apparatus is used widely in a field of diagnostic imaging. Upon measuring a three-dimensional region of the test subject by the X-ray CT, it is general to perform helical scanning in which the test subject is moved in the direction of the rotation axis of the imaging system, while the imaging is performed. The helical scanning is advantageous because it allows measurement within any range in the rotation axis direction. However, upon imaging the heart that needs cardiac gating, there is a problem that it is difficult to bring cardiac beats into sync with the movement of the test subject. On the other hand, in recent years, with the progress of a multi-row structure of the X-ray detector, it is now possible to measure a three-dimensional CT image, over a wide range in the rotation axis direction, by just one revolution of the imaging system. As a result, currently, an apparatus is put into practical use, which allows a scan of the test subject being kept static (hereinafter, referred to as static scanning) and measures a three-dimensional CT image of the entire heart.
In the three-dimensional CT image measured by the static scanning, there is a problem that cone-beam artifact may occur, with getting nearer both ends in the rotation axis direction, and therefore an image quality may be deteriorated. In addition, in proximity to both ends, there are some regions being irradiated with X-rays, other than the region which is able to be reconstructed as a CT image. Therefore, there is a problem that the test subject may be exposed to ineffective radiation. In order to solve the aforementioned problems, a multi-source CT utilizing multiple X-ray sources is suggested by the Non Patent Document 1, and so on.
As another example of the multi-source CT, there is an example described in the Patent Document 3, and it is already put into practical use. In this example, there are prepared two pairs of X-ray source and X-ray detector, and they are placed in such a manner as displaced by 90 degrees with respect to each other, in the rotation angle direction of the imaging system. With this configuration, imaging from the whole circumferential direction of the test subject is possible at a small rotation angle of the imaging system, and therefore there is an advantage that imaging speed can be enhanced. It is to be noted that in this example, since the focal positions of the two X-ray sources are arranged at an identical point in the rotation axis direction, it is not possible to obtain an effect to reduce the cone beam artifact simultaneously with preventing occurrence of exposure to ineffective radiation in the static scanning.
The following patent document 1 describes an example of a method for moving the position of the X-ray focus in the rotation axis direction of the imaging system. This example prevents a shift of the imaging system position in the rotation axis direction, due to thermal expansion of rotating anode of the X-ray tube. An auxiliary X-ray detector placed in proximity to the X-ray tube measures a shift quantity of the aforementioned X-ray focus position, and the entire X-ray tube is moved according to thus measured shift quantity, and the position is modified so that the shift quantity becomes zero. This keeps the position of X-ray irradiation field to be constant with respect to the X-ray detector, and therefore, there is an advantage that it is possible to prevent occurrence of exposure to ineffective radiation and deterioration of image quality in a CT image.
The following patent document 2 describes another method for preventing occurrence of exposure to ineffective radiation due to the shift of the X-ray focus position, and deterioration of image quality of a CT image. In this example, the position of a collimator that restricts the X-ray irradiation field is modified according to the shift quantity of the X-ray focus position, thereby keeping the position of the X-ray irradiation field to be constant with respect to the X-ray detector.